Radio communication systems have been widely used today, including cellular phone systems and wireless local area networks (wireless LANs). Further, the development of a high-speed and large-capacity radio communication system is being advanced so as to respond to the increase in access to the Internet using a radio communication terminal such as a cellular phone handset or use of a music and image distribution service. For example, in a 3rd generation partnership project (3GPP) which performs standards work of cellular phone systems, specifications of a next-generation cellular phone system such as a long-term evolution (LTE) are examined.
Here, as one technique for realizing a high-speed and large-capacity mobile communication system, the multi-antenna transmission techniques for transmitting a transmit signal wirelessly using a plurality of antennas are used. As the multi-antenna transmission techniques, various methods are considered depending on how a plurality of antennas are used. For example, there are considered a diversity system for transmitting a signal with the same content from a plurality of antenna to improve radio communication quality, a multiple input multiple output (MIMO) multiplex system for transmitting a signal with different content from each antenna to improve a transmission rate, and a beam forming system for outputting a directional beam from each antenna toward the direction in which a receiving apparatus is present to improve an antenna gain.
When multiplexing a transmit signal to a plurality of receiving apparatuses using a plurality of antennas, a transmitting apparatus weighs a transmit signal to each receiving apparatus by a column (weight vector) of coefficients for adjusting a phase or amplitude, thereby multiplexing it using a part or the entire of the plurality of antennas. In this case, preferably, the transmitting apparatus dynamically allocates a weight vector to the receiving apparatus at the time of the transmission so that the communication quality may be satisfactory. The throughput of the entire radio communication system is improved by allocating the weight vector to the receiving apparatus in consideration of communication states of the plurality of receiving apparatuses, the transmitting apparatus makes a contribution to improving a throughput of the entire radio communication system.
As a method for controlling the allocation through the transmitting apparatus, there is disclosed an open loop control method in which the transmitting apparatus performs an estimation process of the communication quality of a link (reverse link) to the transmitting apparatus from each receiving apparatus and autonomously determines the allocation (see, for example, Japanese. Laid-open Patent Publication No. 2003-235072). Also, there is disclosed a closed loop control method in which each receiving apparatus performs an estimation process of the communication quality of a link (forward link) to each receiving apparatus from the transmitting apparatus and transmits feedback data to the transmitting apparatus, and the transmitting apparatus determines the allocation based on the feedback data (see, for example, Japanese Laid-open Patent Publication No. 2006-81162).
As a method for controlling an allocation of a weight vector through a transmitting apparatus, a method for determining the allocation based on feedback data from each receiving apparatus is excellent in terms of maintaining a preferable communication quality. The reason is that the communication quality of a forward link and that of a reverse link are not necessarily corresponded to each other.
However, there is a problem that in a conventional technique as disclosed in the Patent Document 2, a processing burden at the time of determining the allocation is large. The transmitting apparatus preferably selects a combination of the optimal weight vectors capable of suppressing interference between transmit signals from among a plurality of candidates of the weight vector to allocate the weight vector to each receiving apparatus. In other words, each receiving apparatus needs to perform an estimation process of the communication quality with respect to the plurality of candidates of the weight vector so that the transmitting apparatus can appropriately determine the allocation. For the purpose of performing an estimation process of the communication quality, there is a problem of increase in a processing burden, circuit size and power consumption of each receiving apparatus. In addition, the amount of data of the feedback data from each receiving apparatus to the transmitting apparatus also becomes large and therefore, is turned to be an oppression factor of a communication band of the reverse link.